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The bark is composed of two substances; the paren- that of the scorzonera, dandelion, &c. condenses into a Plent chyma or pulp, which is the principal part, and a few gum. woody fibres. The parenchyma is exceedingly porous, The situation of the vessels is various. In some plants and has a great resemblance to a sponge ; for it sbrivels they stand in a ring or circle at the inner edge of tbe considerably when dried, and dilates to its former di- bark, as in asparagus; in others, they appear in lines or mensions when infused in water. These pores or ves- rays, as in borage ; in the parsnep, and several other sels are not pervious, so as to communicate with each plants, they are most conspicuous towards the outer edge other;
but consist of distinct little cells or bladders, of the bark; and in the dandelion, they are disposed in scarcely visible without the assistance of the microscope. the form of concentric circles. In all roots, these cells are constantly filled with a thin The wood of roots is that part which appears after the watery liquor. They are generally of a spherical figure ; bark is taken off, and is frmer and less porous than the though in some roots, as the bugloss and dandelion, they bark or pith. It consists of two distinct substances, viz. are oblong. In many roots, as the horse-radish, peony, the pulpy or parenchymatous, and the ligneous. The asparagus, potato, &c. the parenchyma is of one uni- wood is connected to the bark by large portions of the form structure. But in others it is more diversified, bark inserted into it. These insertions are mostly in the and puts on the shape of rays, running from the centre form of rays, tending to the centre of the pith, which towards the ircumference of the bark. These rays are easily discernible by the eye in a transverse section sometimes run quite through the bark, as in lovage; of most roots. These insertions, like the bark, consist and sometimes advance towards the middle of it, as in of many vessels, mostly of a round or oval figure. melilot and most of the leguminous and umbelliferous The ligneous vessels are generally disposed in collates plants. These rays generally stand at an equal distance ral rows running longitudinally through the root. Some from each other in the same plant ; but the distance of these contain air, and others sap. The air-vessels are 'varies greatly in different plants. Neither are they of so called, because they contain no liquor. These airequal sizes : in carrot they are exceedingly small, and vessels are distinguished by being whiter than the others. scarcely discernible ; in melilot and chervil, they are The pith is the centrical part of the root. Some roots thicker. They are likewise more numerous in some have no pith, as the stramonium, nicotiana, &c.; others plants than in others. Sometimes they are of the same have little or none at the extremities of the roots, but thickness from one edge of the bark to the other; and have a considerable quantity of it near the top. The some grow wider as they approach to the skin. The pith, like every other part of a plant, is derived from vessels with which these rays are amply furnished, are the seed; but in some it is more immediately derived supposed to be air-vessels, because they are always from the bark : for the insertions of the bark running in found to be dry, and not so transparent as the vessels betwixt the
rays of the wood, meet in the centre, and which evidently contain the sap.
constitute the pith. It is owing to this circumstance, In all roots there are ligneous vessels dispersed in dif- that, among roots which have no pith in their lower ferent proportions through the parenchyma of the bark. parts, they are amply provided with it towards the top, These ligneous vessels run longitudinally through the as in columbine, lovage, &c. bark the form of small threads, which are tubular, as The bladders of the pith are of very different sizes
, is evident from the rising of the sap in them when a and generally of a circular figure. Their position is root is cut transversely. These ligneous sap-vessels do more uniform than in the bark. Their sides are not not run in direct lines througb the bark, but at small mere films, but a composition of small fibres or threads ; distances incline towards one another, in such a man- which gives the pith, when viewed with a microscope, ner that they appear to the naked eye to be inoscula- the appearance of a piece of fine gauze or net-work. ted; but the microscope discovers them to be only con- We shall conclude the description of roots with obtiguous, and braced together by the parenchyma. serving, that their whole substance is nothing but a These braces or coarctations are very various both in congeries of tubes and fibres, adapted by nature for the size and number in different roots; but in all plants absorption of nourishment, and of course the extension
Plate they are most numerous towards the inner edge of the and augmentation of their parts. bark. Neither are these vessels single tubes ; but, Fig. 9. A transverse section of the root of worm
fig. like the nerves in animals, are bundles' of 20 or 30 wood as it appears to the naked eye. small contiguous cylindrical tubes, which uniformly run Fig. 10. A section of fig. 9. magnified. AA, the from the extremity of the root, without sending off any skin, with its vessels. BBBB, the bark. The round branches or suffering any change in their size or shape. holes CCC, &c. are the lymph-ducts of the bark: All
In some roots, as parsnep, especially in the ring next the other holes are little cells and sa p-vessels. DDD, the inner extremity of the bark, these vessels contain a parenchymatous insertions from the bark, with the cells, kind of lymph, which is sweeter than the sap contained &c. EEEE, the rays of the wood, in which the boles in the bladders of the parenchyma. From this circum- are the air.vessels. N. B. This root has no pith. stance they have got the name of lymph ducts.
3. Of the Trunk, Stalk, or Stem.] In describing These lymph-ducts sometimes yield a mucilaginous the trunks of plants, it is necessary to premise, that lymph, as in the comfrey; and sometimes a white whatever is said with regard them applies equally to milky glutinous lymph, as in the angelica, sonchus, bur- the branches. dock, scorzonera, dandelion, &c. The lymph-ducts are The trunk, like the root, consists of three parts, viz. supposed to be the vessels from which the gums and bal- the bark, wood, and pitb. sams are secerned. The lymph of fennel, when expo- stantially the same in the trunk as in the root, are in sed to the air, becomes a clear transparent balsam ; and many cases very different in their texture and appearance.